Tell you what . . they are good these VW's!!! They like to keep you busy!
I would definately check the main earths from battery to body and especially the engine earths, I had the same problem a couple of years ago when the main earth corroded badly, the engine earth subsequently overheated, and I found that my clutch cable became the earth and promptly melted not good. Remove the earths, make sure the contacts/solder in the cables are still good, if not buy new from an Auto Electrician, clean all contact points with a wire brush, use a bit of copperslip to protect it and bobs your uncle.